Fake preprocessing for any threshold with Shamir secret sharing.

Machines/ShamirMachine.h

@@ -17,8 +17,10 @@ public:
     int nparties;
     int threshold;
 
-    ShamirOptions();
+    ShamirOptions(int nparties = 3, int threshold = 1);
     ShamirOptions(ez::ezOptionParser& opt, int argc, const char** argv);
+
+    void set_threshold(ez::ezOptionParser& opt);
 };
 
 class ShamirMachine : public ShamirOptions

Machines/ShamirMachine.hpp

@@ -39,8 +39,8 @@ ShamirOptions& ShamirOptions::s()
     return singleton;
 }
 
-ShamirOptions::ShamirOptions() :
-        nparties(3), threshold(1)
+ShamirOptions::ShamirOptions(int nparties, int threshold) :
+        nparties(nparties), threshold(threshold)
 {
 }
 
@@ -66,6 +66,12 @@ ShamirOptions::ShamirOptions(ez::ezOptionParser& opt, int argc, const char** arg
     );
     opt.parse(argc, argv);
     opt.get("-N")->getInt(nparties);
+    set_threshold(opt);
+    opt.resetArgs();
+}
+
+void ShamirOptions::set_threshold(ez::ezOptionParser& opt)
+{
     if (opt.isSet("-T"))
         opt.get("-T")->getInt(threshold);
     else
@@ -80,7 +86,6 @@ ShamirOptions::ShamirOptions(ez::ezOptionParser& opt, int argc, const char** arg
         cerr << "Threshold has to be positive" << endl;
         exit(1);
     }
-    opt.resetArgs();
 }
 
 template<template<class U> class T>

Protocols/ShamirShare.h

@@ -50,7 +50,11 @@ public:
 
     static string type_short()
     {
-        return "S" + string(1, clear::type_char());
+        auto res = "S" + string(1, clear::type_char());
+        auto opts = ShamirOptions::singleton;
+        if (opts.threshold != (opts.nparties - 1) / 2)
+            res += "T" + to_string(opts.threshold);
+        return res;
     }
     static string type_string()
     {

Protocols/fake-stuff.hpp

@@ -187,7 +187,7 @@ void make_share(ShamirShare<T>* Sa, const V& a, int N,
   {
       auto& share = Sa[i];
       share = a;
-      for (int j = 0; j < (N - 1) / 2; j++)
+      for (int j = 0; j < ShamirOptions::singleton.threshold; j++)
           share += vandermonde[i][j] * randomness[j];
   }
 }

Utils/Fake-Offline.cpp

@@ -34,6 +34,7 @@
 #include "Math/Z2k.hpp"
 #include "Math/gfp.hpp"
 #include "GC/Secret.hpp"
+#include "Machines/ShamirMachine.hpp"
 
 #include <sstream>
 #include <fstream>
@@ -615,6 +616,16 @@ int main(int argc, const char** argv)
         "-s", // Flag token.
         "--special" // Flag token.
   );
+  opt.add(
+          "", // Default.
+          0, // Required?
+          1, // Number of args expected.
+          0, // Delimiter if expecting multiple args.
+          "Number of corrupted parties for Shamir secret sharing "
+          "(default: just below half)", // Help description.
+          "-T", // Flag token.
+          "--threshold" // Flag token.
+  );
   opt.parse(argc, argv);
 
   int lgp;
@@ -680,6 +691,12 @@ int FakeParams::generate()
     return 1;
   }
 
+  if (nplayers > 2)
+  {
+    ShamirOptions::singleton.nparties = nplayers;
+    ShamirOptions::singleton.set_threshold(opt);
+  }
+
   int ntrip2=0, ntripp=0, nbits2=0,nbitsp=0,nsqr2=0,nsqrp=0,ninp2=0,ninpp=0,ninv=0, nbittrip=0, nbitgf2ntrip=0;
   vector<int> list_options;
   int lg2, lgp;